The Fischer-Tropsch technology is known in scientific literature for preparing hydrocarbons from gas mixtures based on hydrogen and carbon monoxide, conventionally known as synthesis gas. A compendium which summarizes the main works on the Fischer-Tropsch synthesis reaction is contained in Bureau of Mines Bulletin, 544 (1955) entitled “Bibliography of the Fischer-Tropsch Synthesis and Related Processes” H. C. Anderson, J. L. Wiley and A. Newell.
In general, the Fischer-Tropsch technology is based on slurry reactors, which are normally used in relation to chemical reactions which take place in three-phase systems, wherein a gaseous phase is bubbled into a suspension of a solid in a liquid. The gaseous phase consists of the synthesis gas with a molar ratio H2/CO varying from 1 to 3, the dispersing liquid phase, at high temperature, represents the reaction product, i.e. linear hydrocarbons mainly with a high number of carbon atoms, and the solid phase consists of the catalyst.
The reaction product which is discharged from the reactor therefore consists of a suspension containing particle, which are also fine, that must be treated to separate the solid (catalyst) from the liquid phase. Whereas the catalyst is recycled to the synthesis reactor, the liquid is subjected to subsequent upgrading treatment, for example hydrocracking and/or of hydro-isomerization, to obtain hydrocarbon fractions of industrial interest.
European patent 609,079 describes a reactor for Fischer-Tropsch reactions consisting of a gas bubble tower containing a suspension made up of catalyst particles suspended in the liquid hydrocarbon. The synthesis gas is fed to the bottom of the reactor, whereas the synthesized hydrocarbon is recovered at the head.
In order to avoid the entrainment of catalyst particles, the reactor is equipped with cylindrical filtration devices internally arranged in the upper part.
International patent application WO 97/31693 describes a method for separating a liquid from a suspension of solid particles which comprises, in a first phase, degasifying the suspension and, in a second phase, filtering the suspension through a tangential flow. In particular, the suspension comes from a Fischer-Tropsch reactor and consists of synthesized heavy hydrocarbons which entrain the catalyst particles.
Other examples of methods for separating the catalyst contained in the suspension leaving a Fischer-Tropsch reactor are described in European patent 592,176, international patent application WO 94/16807, English patent 2,281,224, U.S. Pat. Nos. 4,605,678 and 5,324,335 and German patent 3,245,318.
A disadvantage associated with Fischer-Tropsch processes, for example with those mentioned above, and in particular Fischer-Tropsch processes in which the catalyst is based on cobalt, is that a hydrocarbon mixture is produced, which, in the subsequent transformation phases (hydro-isomerization and/or hydrocracking), causes an alteration in the performances of the corresponding catalysts.